In the field of motor vehicles, the purpose of an alternator is to generate a direct voltage for the supply of the on-board electrical network, and to maintain it at a predetermined set value, for the electrical supply to the vehicle equipment and to a battery.
The alternator comprises a stator which is secured to the structure of the vehicle, and surrounds a rotor supported by a shaft which is rotated by the thermal engine of the vehicle by means of an alternator belt. The alternator makes it possible to transform a movement of rotation of the rotor into an electric current induced in the stator windings. Downstream from the alternator, an alternating/direct converter, which for example is reversible, then makes it possible to generate a direct voltage for the supply to the on-board electrical network.
Upstream from the alternator, a regulating system, known as the “regulator”, which is generally integrated in the alternator, is used to maintain a stable voltage at the output from the alternator, independently from the speed of rotation of the engine or the electric consumption of the vehicle's equipment. For this purpose it controls the production of an excitation current supplied to the inductor rotor, in order to control the intensity of the associated magnetic field, and thus the voltage induced by this magnetic field and the current produced at the output from the alternator, which, by means of the battery and charges, determines the voltage of the on-board network.
Nowadays, parts manufacturers in the motor vehicle industry have developed very high-performance alternators by implementing digital techniques. Regulating systems are known wherein the voltage produced at the output from the alternator is sampled and digitally converted, the digital values obtained then being compared continuously with a set value, for example 14 V. After filtering, the result of this comparison is used to quantify the intensity of the excitation current to be conveyed to the alternator, in order to maintain the output voltage at the set value.
As previously described, the alternator is coupled mechanically to the thermal engine. However, in thermal engines, a phenomenon known as acyclism occurs, caused by the explosions in the cylinders of the thermal engine, which give rise to accelerations and decelerations around the mean speed. This takes the form of non-uniform speeds of rotation of the engine which affect the alternator.
The curve which is representative of the current output according to the speed of rotation for a standard alternator can, with an approximate approach, be modeled simply by two straight lines, i.e. a first straight line with a steep gradient which for low speeds describes the beginning of increase of the current, and a second straight line with a slight gradient, for higher speeds. When the speed of rotation of the alternator is higher than the speed of rotation corresponding to the joining point of the two straight lines (and which is known as the bottom-of-curve speed Vbc), the acyclism gives rise to little current variation, and therefore to little variation of the voltage produced by the alternator (FIG. 1). On the other hand at a very low speed, when the speed of rotation of the alternator is lower than this bottom-of-curve speed (corresponding to the joining point of the two straight lines), the acyclism oscillations give rise to greater fluctuation of the current output from the alternator (FIG. 2), which involves substantial variations of the voltage at the level of the battery.
In parallel, mainly for reasons of energy-saving, certain motor vehicle manufacturers have been induced to lower the idling speed of the thermal engines which equip their vehicles, to speeds of rotation lower than 800 rpm. Since the alternator is coupled mechanically to the thermal engine, this means that the alternator has a speed of rotation which decreases proportionally during these idling phases. In these situations, the case illustrated by FIG. 2 exists, in which the phenomenon of acyclism gives rise to strong variations of the current output by the alternator, and consequently to strong variations of the on-board network voltage. These strong voltage variations are taken into account by the regulator, which tends to oppose this effect by means of regulation, with the result of making certain functions inoperative.